Hot water at cold tap: questions about recirc pump, sprinklers, and check valves

I recently had an under-sink Metlund D'mand hot water recirculation pump installed, using the cold water line as the return. Normally it works fine. However, I noticed that if I run my lawn sprinklers, I get hot water out of the cold tap, implying the hot water is being sucked through the pump to the cold side. The feed to the irrigation is right near the bathroom where the pump is installed.

I contacted the manufacturer and they said the drop in pressure when the sprinklers are on is overcoming the 20 psi rating of the pump's IFC valve and it is opening allowing the hot water to go through. They suggested putting a 25 psi spring-loaded check valve on the cold side to overcome this problem. I am not a plumber, but I have a technical background, and want to understand this issue and solution a bit better. I am seeking answers to the following:

1) I'm guessing the 20 psi rating on the pump's IFC valve means that if the pressure differential between the hot and cold sides goes over 20 psi the valve opens - is this correct? And what does IFC stand for? Internal Flow Control?

2) The 25 psi rated spring-loaded check valve should be installed with the flow direction being pump => cold water line, correct? Can't see how it would work otherwise, but want to be sure.

3) How does the proposed solution work? Do the two psi ratings add up because they are in series and therefore the differential needs to be 45 psi in order for both valves to open?

Well, no replies so far; not sure if I'm asking too stupid questions, or if I violated forum etiquette, or no one really knows the answers. Anyways, I tried installing a spring-loaded check valve on the cold water side of the pump, with the direction of the valve going away from the pump. It made no difference. I think I have the installation correct. I bought the valve at Pace Supply and asked about the psi rating of the valve but they had no idea. Seemed like a 3/4" spring-loaded check valve only came in one flavor.

This situation isn't very good because I'm watering my lawn with warm water. If anyone has any ideas/suggestions it would be greatly appreciated!

I doubt that the pump can overcome a 25 psi check valve, (I would have to see a pump flow chart), and also doubt that the Metlund one is 20 psi. But even if you did do it, it might be ineffective, because the check valve would have to allow flow in the direction it is already going, which is from the hot side to the cold. Otherwise, it would NOT allow the pump to circulate the water properly. In addition, there should be a thermostatic valve in the pump that shuts off the flow when the temperature increases to PREVENT that from happening.

Thanks hj. I did install the check valve, and your suspicion is correct: it is ineffective.

I don't know if there is a thermostatic check valve in the pump. The pump stops running once it detects a rise in water temp. so there is some sort of temp sensor, but apparently water can still be sucked thru if there is a large pressure drop on the cold side. The best I can understand it is that smaller drops such as when a cold tap is on aren't enough to overcome the pump's 20 psi IFC (whatever that is) but a large drop (e.g. irrigation) allows water to flow thru the pump. It almost seems like a design flaw in the D'MAND system, but I don't know enough about pump design to say that definitively.

The manufacturer also suggested two alternative solutions:

1) Install a wax valve on the cold side. This valve would presumably shut after ~15 sec. of hot => cold flow and help solve this issue. However, I am skeptical, because what happens if the valve is shut, and hasn't opened when someone activates the recirc pump? Risk of pump damage?

2) Install a mechanical zone valve on the cold side that would be wired to the pump's electronics. Not sure about the details, but I'm guessing this would be a valve that would close when the pump is not active and open when it is active (which is functionality I thought would've already been designed into the D'MAND system, but again, I don't know pump design). This seems like it would work, but it is pricey (estimate $170) and most complicated to install.

Man, what a headache. Had I known about the issues I would have with this (there were others prior) and the costs, I would never have had it installed. Yes, I had it professionally installed. ~$1K into this and still not happy. And the plumber who installed it hasn't got back to me either after I forwarded the manufacturer's email to him asking for his opinions.

Just wanted to report back in case anyone else also ends up with this situation. I ended up installing a zone valve (#2 in above post) and that solved the problem. The manufacturer supplied the install instructions. It was a bit of a chore connecting the zone valve to the pump's electronics as I had to take things apart and get special "z-clips" (manufacturer kindly sent me these) to connect the wires. In the process I accidentally shorted the power for a split second, but it was enough to fry the control circuit board, which added another $200 bill to this project to get a replacement. I later learned after probing the circuit that the zone valve electrical connection can be made in a far simpler manner without having to deal with 'z-clips' or even taking apart the control module. I can supply details if anyone asks.